The present invention relates to a novel process for the preparation of 2,2',4,4',6,6'-hexanitrobibenyl (HNB). This compound when reacted with an alkaline metal hypochlorite will form the compound 2,2',4,4',6,6'-hexanitrostilbene, an important thermally-stable explosive which is also useful as a nucleating agent promoting a desired mode of crystallization of trinitrotoluene.
Up until the present invention, the preferred method for preparing HNB was described in U.S. Pat. No. 3,505,413 and in a paper by Shipp and Kaplan, Journal of Organic Chemistry, 31, 857 (1966). The references teach dissolving trinitrotoluene (TNT) in a solvent mixture comprising one part tetrahydrofuran and two parts methanol by volume. A solution of sodium hypochlorite, which is made alkaline by the addition of sodium hydroxide is then slowly added to the solution of TNT. During the addition of the sodium hypochlorite to the solution of TNT, the temperature did not exceed 35.degree. C. After a suitable aging period (15 minutes), the crystalline material was filtered and then washed with methanol and dried. It is reported that the amount of crude HNB obtained is 79% of theoretical.
The prior art process, while effective, does have a decided disadvantage in requiring the use of large volumes of solvent mixtures, which include expensive compounds. U.S. Pat. No. 3,505,413 explains that the solvent should be one which will dissolve TNT at ambient temperatures and below and which, with the addition of a lower aliphatic alcohol, will provide a homogenous solution with an alkaline metal hypochlorite. Examples of such solvents are tetrahydrofuran, p-dioxane, diglyme and acetonitrile, which are all water-miscible materials.
Also, in accordance with the prior art process, the temperature at which the reaction proceeds must be kept low in order to minimize undesired side reactions between TNT and alkali as well as the competing reacton of the solvent system. The Shipp et al. paper suggests temperatures under 15.degree. C.
Unlike the prior art process, it has now been discovered that HNB can be prepared by using a solvent system which is relatively inexpensive and will permit the reaction to proceed at temperatures higher than was originally thought possible and produce a crude HNB product of lighter color and greater purity.